1. Field of Invention
This invention relates to phase interferometer radars, and more particularly to means for resolving the ambiguity in interferometer elevation angle measurement.
2. Description of the Prior Art
One type of phase interferometer radar known to the prior art measures elevation angle to terrain as a function of range ahead of an aircraft so as to provde a profile of the terrain over which the aircraft will fly. This is done by measuring the instantaneous difference in phase between the energy received in two antennas which are displaced relative to each other in a direction perpendicular to the boresight (essentially vertical in this case).
The received energy is that portion of a short pulse, transmitted from either a separate antenna or one or both of the receiving antennas, which is reflected by the terrain. The time of arrival is a measure of the slant range to that portion of the terrain from which the reflected energy is received and the corresponding phase difference is a measure of the elevation angle.
The relationship between phase angle and elevation angle is EQU .phi.=2.pi.d/.lambda.sin .theta.
where
.phi.=phase angle in radians PA1 .lambda.=wavelength PA1 .theta.=elevation angle relative to boresight in radians PA1 D=distance between the two antennas PA1 D.phi./d.theta.=2.pi.d/.lambda.cos .theta.
If the limits of phase angle measurement are .+-..pi. and d/.lambda.=1/2, then the corresponding limits of .theta. are .+-.90.degree. . Assuming that the antennas receive no energy from angles greater than 90.degree. , then in this case there would be a unique relationship between phase angle with respect to elevation angle. The rate of change of phase angle with respect to elevation angle would be .pi. at the boresight and would be zero at the extremes.
In practice the range of elevation angles which is of interest is much less than .+-.90.degree. , and in order to achieve maximum accuracy of measurement it is desirable to have as large a scale factor, or rate of change of phase angle with respect to elevation angle as practicable. To achieve this, the separation of the antennas must be greater than one wavelength. For example, in one known radar d/.lambda.=1.44 and therefore EQU .phi.=2.88.pi.sin .theta. EQU .phi.=.+-..pi.when sin .theta.=.+-.1/2.88
or EQU .theta.=.+-.20.3 degrees
If energy is received from an elevation angle beyond these limits, its direction will be indistinguishable by phase measurement alone from energy coming from a point within the limits. For example, if sin .theta.=2/2.88 or .theta.=-44 degrees, then .phi.=-2.pi.which is indistinguishable from .phi.=0.
Techniques known to the art for accepting some data as coming from within the desired range of elevation angles and rejecting other data as coming from outside that range, are complex logical processes which rely on either detecting the "flip" when the phase angle changes from +.pi. to -.pi. or on some assumptions regarding the terrain. Such methods are not completely reliable for all conditions and there are some situations in which terrain actually more than 20.3 degrees above the boresight (for instance) is made to appear considerably below boresight.